1. Field of the Invention
The present invention, in general, relates to a composition for, and a method of, making a highly porous anti-flection coating, and, more particularly, to an optical anti-reflection coating for simultaneously improving the transmission and preventing or minimizing undesired reflections of visible and other electromagnetic radiation of the kind occurring, for instance, at cathode ray tubes, liquid crystal displays, instrument panels, spectacle lenses, picture tubes, solar collectors and automotive windscreens.
2. The State of the Art
When light is penetrating the interface between two media of different diffractive indices a portion of the radiation is reflected. The reflected portion of light vertically impinging upon a pane of glass having an index of refraction n=1.5 is about 4%. If, however, the light falls on the interface at an acute angle, a much greater portion is reflected.
Display devices such as, for example, cathode ray tubes or liquid crystal displays are used for many different applications. While the quality of their images has been improved, the images are often difficult to distinguish because of reflections. Furthermore, the reliability of the information transmitted, for instance, from instrument panels, watch crystals or automotive windscreens is often reduced significantly because of reflections.
Reduced reflections are desirable for a great many optical systems. Anti-reflection coatings on spectacle lenses are a well-known example. By using covers with anti-reflection coatings on solar devices their efficiency could be enhanced.
Reflection may also be reduced by the application of interference coatings. If a coating of a thickness .lambda./4 is applied to glass (n=1.5), for instance, destructive interference will result between the reflected portion at the interfaces between air and coating as well as coating and glass. Yet the conditions for destructive interference always hold only for a particular wave length and a particular angle of incidence. The refractive index of the coating determines the level of minimum reflection. For glass of optimum anti-reflection properties it has to be 1.22 in order to result in close to zero at a wavelength of .lambda.. Such a low index of refraction cannot, however, be achieved with dense coatings. A single anti-reflection coating of the kind mentioned is extremely effective for visual as well as for solar purposes. The antireflective effect of the often used treble layered interference coatings reasonably extends over the range of visible light, i.e. from about 400 nm to about 800 nm. Such coatings are, however, unsuited for solar applications because the spectrum of solar radiation covers a much wider range.
Reduced reflection may also be achieved by a surface with a graduated refractive index. That is to say, rather than changing abruptly the refractive index approaches the value for glass from the value for air (n=1) in several steps. The advantage of such a layer is that it effectively reduces reflection over a broad-band spectral range and for all angles of incidence.
Different processes for fabricating anti-reflective surfaces exist already. For example, transparent anti-reflection films or coatings of different refractive indices have been applied to surfaces to reduce undesirable surface reflections. Such processes do, however, entail problems as they involve the application of at least two coatings. This results in lower productivity and higher production costs. To render spectacle lenses or video screens anti-reflective, for instance, interference layers are applied by vapor deposition. While the layers thus produced are relatively abrasion resistant, their high costs of about $60.00 (DM 100.00) to about $100.00 (DM 150.00) per square meter of treated surface area constitute a disadvantage. On the other hand, sputtered multi-layered interference coatings are suitable only for visual anti-reflection coatings and cannot be applied to every kind of substrate geometry, such as the internal surfaces of tubes, for instance. Diffuse anti-reflection by roughening the surface is most common but does not result in enhanced transmission. Anti-reflection of high optical value may be obtained by a treble-layered system involving a sol-gel process. Such a process is suitable even for making large display windows anti-reflective. But at a cost of about $100.00 (DM 140.00) to about $150.00 (DM 200.00) it is a relatively expensive process.
German patent application 4,430,859 discloses an anti-reflection coating consisting of two layers of different indices of refraction. At least its first layer also contains a light absorber. While the processes for producing these layers are conventional, involving sputtering, vapor deposition or other common coating processes, they are, nevertheless, expensive. Moreover, another disadvantage of such an anti-reflection coating and of its manufacturing process resides in the fact that because of the light absorber it is very unlikely that the transmission is significantly increased.
European patent specification 0,514,973 discloses an anti-reflection coating for cathode ray tubes, in particular. The coating has a graduated index of refraction decreasing from the surface of the substrate in the direction of the coating surface. The coating is produced by a sol-gel process in which the conditions of the reaction during the gel formation are varied such that the resultant gel is non-porous with the degree of cross-linkage increasing from the surface of the substrate toward the outer surface of the coating. The disadvantage of this process is the dispersion occurring as a result of relatively large particles in the coating. The dispersion results in reduced transmission thus rendering it unsuited for solar applications. Also, the multiple coating application is very expensive.
U.S. Pat. No. 4,830,879 also teaches an anti-reflective coating for surfaces of glass, metal and crystal. The coating is produced by multiple coating and by a sol-gel process by hydrolytic condensation of metal alkoxides. To this end, four differently aged solutions each containing particles of a size different from those of the other solutions are produced for consecutive coatings of a substrate. The resultant antireflection coating has a gradient of particle sizes and, hence, of porosity and refractive index. While such multi-layer coatings or gradient layers lead to broad-band antireflection, they do, however, depend upon extremely porous structures (n=1.05!) in the direction of the surface of the coating. These structures tend to be very unstable mechanically. The disadvantage of this kind of coating, moreover, resides in its complex manufacturing process which, because of the numerous process steps, is very expensive.